NATIONAL HARBOR, MD. — The F-35 is no longer a trillion-dollar program and costs are likely to continue to drop, officials for the Joint Strike Fighter program said Tuesday.

US Air Force Lt. Gen. Christopher Bogdan, the head of the F-35 Joint Program Office, confirmed that his office estimates the sustainment cost for the F-35 has decreased to $857 billion, a significant drop from a figure often cited as $1.1 trillion.

That larger figure was from a three-year-old government report, Bogdan said at the Air Force Association’s Air & Space Conference, and the new number reflects a more accurate assessment of the program. However, given the dangers of trying to predict costs and inflation over a 50-year period, even that number contains “a lot of assumptions.”

Speaking earlier in the day, Lorraine Martin, Lockheed Martin’s executive vice president and general manager of the F-35 Lightning II Program, said the cost for an F-35A conventional takeoff and landing variant has dropped to less than $100 million per aircraft.

“That’s a great milestone for us,” Martin said, noting that in negotiations from low-rate initial production (LRIP) 1 aircraft to LRIP-5, costs per airframe dropped 55 percent. “We are dragging costs down across the entire program.”

Martin and Bogdan both expressed confidence that previously reported issues are being addressed and will not cause further program delays.

One long-term concern has been the capability of the F-35 pilot’s helmet, required for interfacing with the fighter’s high-tech electronics. Lockheed’s helmet is suitable for initial operating capability (IOC), Martin said, but the company is installing an enhanced night-vision camera that would be put in place by the time LRIP-7 models enter production.

The tailhook on the Navy’s F-35C carrier variant has also been a running concern, but Lockheed is confident its new design has solved the problem. The tailhook will be tested in October and November with trap runs at Naval Air Engineering Station Lakehurst in New Jersey, with carrier tests due next summer.

Another concern has been the plane’s ability to fly near lightning, which is currently restricted due to a flaw in the On-Board Inert Gas Generating System (OBIGGS), used to prevent a fuel-tank explosion in case of a lightning strike. A fix has gone through a critical design review and will be put into place for the LRIP-7 models, with a retrofit option also in development for the older production platforms.

With 10 million lines of code on the plane, the biggest challenge remains software. “It is tricky, it is hard and it is the number one thing that paces the program right now,” Bogdan said.”

The Marine Corps and Air Force will be running the interim block 2B software when they declare IOC in 2015 and 2016, respectively. While Bogdan said he was confident those services will meet their IOC dates with 2B, the development of the more advanced 3F software “heavily depends” on the work that comes before.

“We are addressing and retiring risks,” Martin said. “These are all known issues that we have solutions for. We will continue to focus on them, but we are putting them behind us.”

With those known issues in the process of being fixed, Bogdan has turned his attention to newer issues as they arise.

“There are pieces and parts on this airplane that are simply breaking too much,” Bogdan said, likening the situation to a game of Whac-A-Mole.

When asked for specific examples, Bogdan brought up the tires on the F-35B short takeoff/vertical landing model designed for the Marines.

Tires for other jump-jet designs need more buoyancy to handle the weight of the plane coming straight down to land. But when taking off on a runway, the tires need durability — the opposite characteristic.

Because of that dichotomy, the B model is burning through tires at an unsustainable rate. So Bogdan has asked tire-manufacturer Dunlop to develop a better tire, a process that he said would not cost the US government anything.